The objective of this project is to determine whether early life exposures to persistent organic pollutants contribute to skin barrier defects and enhanced susceptibility to skin disease. The skin epidermal permeability barrier (EPB) forms late in development and is essential for life and health. The EPB prevents dehydration and protects the body against infection, and physical and chemical insults. Our results indicate prominent regulation of keratinocyte differentiation by the aryl hydrocarbon receptor (AHR), acting to affect the formation and barrier function of the epidermis. Furthermore, the epidermal growth factor receptor (EGFR) signaling pathway, an important mediator of epidermal homeostasis, opposes keratinocyte differentiation and blocks AHR-mediated gene expression. The AHR is among a diverse set of transcription factors that regulate EPB function. Because of its activation by a multitude of microbial-, radiation- and plant-derived ligands, the AHR is uniquely poised to sense the environment and initiate cellular adaptive responses. Consistent with this role, recent studies indicate that AHR activation in keratinocytes leads to the expression of barrier forming, bactericidal, immune-modulatory, and detoxication proteins. While most of the naturally occurring AHR agonists induce enzymes that terminate their effect by metabolism, persistent pollutants such as 2,3,7,8- tetrachlorodibenzo-p-dioxin (TCDD) elicits many toxicities including skin disease. Our lab showed that in utero exposure to TCDD accelerates the formation and impairs the function of the EPB, thus revealing a new susceptibility factor of skin disease. In order to understand how the AHR affects keratinocyte differentiation, the role of the AHR in the development of the epidermis, and the importance of the cross-talk between the AHR and epidermal growth factor receptor (EGFR) signaling pathways we propose three specific aims. In aim 1 we will elaborate the mechanism(s) by which the ligand-activated AHR promotes human epidermal keratinocyte differentiation. In aim 2 we will determine the effects of gain or loss of AHR function on the EPB and the assembly of the skin microbiome. In aim 3, we will determine the influence of EGFR signaling on AHR activity and the occurrence and severity of TCDD-induced EPB toxicity. Growing evidence points to early life exposures to pollutants as a contributing factor in disease. The prevalence of skin disease and infection are high, and even greater in children, yet the influence of environmental exposures on this occurrence is largely understudied. Findings from the proposed studies should greatly enhance the understanding of how early life exposures to pollutants can alter skin health.